Signal processor

ABSTRACT

In the signal processor disclosed herein, the respective sets of echo signals reflected back from an environment in response to a radiated signal comprising a pair of adjacent frequency components are modified according to a preselected time-amplitude function and are cross-combined so as to generate an output signal whose amplitude varies as a function of the net displacement of a target within the environment. Further, the output signal is relatively insensitive to moving clutter which does not undergo a substantial net displacement.

United States Patent Pedersen 51 Oct. 3, 1972 [54] SIGNAL PROCESSOR 72 It I N Ped Primary Examiner-T. H. Tubbesing 1 men or g 2 1 mmgtonAttorneyl(enway, Jenney & Hildreth [73] Assignee: Panametrics, lncQ,Waltham, Mass. 57 ABSTRACT [22] Filed: July 2, 1970 21 Appl. No.: 51,845

In the signal processor disclosed herein, the respective sets of echosignals reflected back from an environment in response to a radiatedsignal comprising a pair of adjacent frequency components are modifiedacg 343/5 cording to a preselected time-amplitude function and a n a v at s u I I I I a n as of Search DD, 7.7, 8 whose amplitude varies as afunction of the net placement of a target within the environment.Further, [56] References Cited the output signal is relativelyinsensitive to moving UNlTED STATES PATENTS clutter which does notundergo a substantial net displacement. 3,018,477 l/l962 Brault et a1...343/7.7 3,631,490 12/1971 Palmieri ..343/7.7 6 Claims, 1 DrawingFigure I3 l5 R DUAL FREQUENCY I Z 1) IR FILTER TRANSDUCER i] l le -FILTER j 4| 4e 57 6| 73 I MUL'l'IPLlER -{FILTER MULTIPLIER FILTER IMULTlPLlER i l I 0| FFAERENTIATOR 83 I as J 1 MULTIPLIER -FlLTER I Z-INTEGRATOR| I JDIFFERENTIATOR 43 4? 49 59 e3 1 I OUT u {MULTIPLIERFILTER MULTIPLIER FILTER: MULTIPLIER SIGNAE i I 75 l SIGNAL PROCESSORBACKGROUND OF THE INVENTION This invention relates to a signal processorand more particularly to apparatus for processing the Doppler shiftedcomponents of a pair of echo signals produced in response to atransmitted signal comprising a pair of adjacent frequency components.

In the fields of radar and sonar technology, it has long been recognizedthat echo signals obtained from a moving target can be distinguishedfrom those signals returned from a stationary-background (fixed clutter)by examining the Doppler shifted components of the echo signal. Systemswhich display or recognize only the Doppler shifted echo components arewell known and are typically referred to as moving target indicators(MTI). While such moving target indicators provide good differentiationbetween a moving target and background signals associated withstationary targets, such systems do not provide differentiation orseparation between those targets which undergo relatively large netdisplacements, e.g., those which move relatively steadily on a definitecourse, and those which are merely reciprocating non-periodically ormoving randomly around a generally fixed location. In other words,moving clutter is relatively indistinguishable from a desired target inconventional moving target indicator designs, when the Doppler returnsof the moving target and the moving clutter occupy the same bandwidth.This difficulty with randomly moving clutter has heretoforesubstantially limited the success of various intrusion detection systemswhether they employ electromagnetic energy, i.e., radar systems, oracoustic energy, i.e., sonar systems. I

Among the several objects of the present invention may be noted theprovision of a signal processor which operates on Doppler shifted echosignals and which provides an output signal which is a function of thenet displacement of a target from which echo signals are returned over apreselected integration period; the provision of such a signal processorwhich is relatively insensitive to echo signals produced by a targetwhich moves reciprocally or which moves randomly around a generallyfixed location; the provision of such a signal processor which providesan output signal which varies as a function of the net displacement of atarget substantially independently of its path; the provision of such asignal processor which operates on respective sets of echo signalsreflected back from an environment in response to a radiated signalcomprising a pair of adjacent frequency components; the provision ofsuch apparatus which is relatively insensitive to noise; the provisionof such apparatus which employs synchronous detection and demodulation;the provision of such apparatus which is reliable and which isrelatively simple and inexpensive. Other objects and features will be inpart apparent and in part pointed out hereinafter.

SUMMARY OF THE INVENTION Briefly, apparatus according to the presentinvention is operative to process the Doppler shifted components ofrespective sets of echo signals reflected back from an environment inresponse to a radiated signal which comprises a pair of adjacentfrequency components. The echo signals are detected or demodulated so asto generate respective sets of Doppler information signals havingfrequency components equal in frequency to the respective Doppler shiftfrequencies. Each of the Doppler information signals is thendifferentiated to provide a respective differential signal. A firstproduct signal is generated having an amplitude which is proportional tothe product of the amplitude of one of the Doppler information signalswith the amplitude of the differential signal corresponding to the otherof the Doppler signals. A second product signal is generated having anamplitude which is proportional to the product of the amplitude of theother of the Doppler information signals with the amplitude of thedifferential signal corresponding to the first Doppler informationsignal. An output signal is then generated having an amplitudewhichvaries as a function of the time integral of the difference between theamplitudes of the two product signals.

BRIEF DESCRIPTION OF THE DRAWINGS The single drawing is a block diagramof an echo target sensing system employing a signal processor of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As noted previously, the signalprocessor of the present invention operates on the Doppler informationcontained in the respective sets of echo signals reflected back from anenvironment in response to a radiated signal comprising a pair ofadjacent frequency components. As is understood, the radiated signal andthe echo signals may comprise either electromagnetic energy or acousticenergy depending on the particular use to which the system is to be put.While dual frequency echo interrogation sources have been employedheretofore, these prior uses of two or more frequencies have typicallybeen for different purposes than that for which the present invention isintended and these prior art devices have not operated in the manner ofapparatus according to the present invention. Examples of such prior artsystems are those disclosed in U. S. Pat. Nos. 3,015,819, 3,018,477 and3,165,738.

As means for transmitting dual frequency interrogating energy andreceiving the corresponding echo signals are well known in the art, asexemplified by the patents identified above, such means are notdescribed in detail hereinafter but are described only insofar as isnecessary for an understanding of the signal processing apparatus of thepresent invention.

In FIG. 1 a dual frequency source is indicated generally at 11. Source 11 is assumed to provide signals at two adjacent frequencies,designatedwl and m2 (expressed in radians). For reasons which will beapparent hereinafter, it is preferred that these two components berelatively close in frequency, e.g. within one percent. These two signalcomponents are combined or mixed, as indicated at 13, and the sum signalis applied to a suitable transducer 15 for radiating energy at thesefrequencies into an environment in which it is desired to detect movingtargets. In the case of radar applications, the transducer 15 maycomprise suitable r.f. power amplifiers and an antenna, while in sonarapplications, the transducer may comprise a loudspeaker or otherelectro-acoustic transducer.

Objects or targets in the environment will reflect a portion of theradiated energy back to the apparatus where it is picked up by asuitable transducer, as indicated at 19. As the echo signals receivedwill typically be of relatively small amplitude, these received signalsare passed through a preamplifier, as indicated at 21, so as to beamplified to a level suitable for intermediate processing, as describedhereinafter, to extract the Doppler information contained in thesesignals. While the transmission and reception have been illustrated asbeing performed by separate transducers in the embodiment illustratedfor the purpose of simplicity of explanation, it will be understood bythose skilled in the art that a single transducer or antenna may be usedby employing a suitable circulator or other isolation element to preventan excess of the transmitted power from being introduced directly intothe receiver system.

As will also be understood by those skilled in the art, there will be arespective set of echo signals for each of the transmitted frequencycomponents m1 and (02, each set comprising a component at the originaltransmitted frequency (reflected back from stationary objects) and aplurality of Doppler shifted components reflected back from movingobjects or targets. Since the reflecting object may be moving eithertoward or away from the apparatus, the Doppler components may be eitherabove or below the respective original transmitted frequency. The twosets of echo signals are separated by respective filters 23 and 25 whichhave bandwidths sufficient to pass the respective transmitted frequencycomponent together with its associated Doppler components, but whicheffectively blocks the other of the two transmitted frequencies and itsassociated Doppler components.

Coherent samples of the transmitted frequency components ml and (02,obtained from the source 11, are applied to a multiplier circuit 31 soas to obtain a product signal which includes a component at a frequencywhich is equal to the difference in frequency between the two originaltransmitted components. This product signal is applied to a filter 35which extracts this difference frequency component and the extractedcomponent is amplified as indicated at 36.

Each of the coherent frequency samples obtained from the source 11 isalso applied to a respective multiplier circuit 41 and 43 where each iscombined with or multiplied by the echo signal corresponding to theother of the original transmitted frequencies. In other words, the setof echo signals corresponding to or produced by the original transmittedfrequency 101 are multiplied by the (02 signal while the set of echofrequencies corresponding to the transmitted frequency 102 aremultiplied by the signal of frequency m1. As is understood, each of theproduct signals so generated will have a set of frequency componentscentered nominally on the difference between the two originaltransmitted frequencies. For convenience, this difference frequency maybe considered to be an i.f. (intermediate frequency) of the system. Theproduct signals obtained from the multipliers 41 and 43 are applied tofilters as indicated at 45 and 47, respectively, where the i.f.frequency components are separated out and these separated i.f.components are amplified as indicated at 48 and 49. As indicated, thefilters 45 and 47 are, like the filter 35, centered on the differencefrequency but have bandwidths sufficient to pass Doppler componentsseparated from the i.f. frequency only by the amount of the Dopplershifts.

The substantially pure difference frequency signal obtained from thefilter 35 is employed to synchronously detect or demodulate the two setsof Doppler shift components. For this purpose, the substantially puredifference frequency signal is applied to one input of each of a pair ofmultiplier circuits 57 and 59. One set of echo signals, shifted to thedifference or i.f. frequency, is applied to the other input of eachmultiplier.

The product signals obtained from the multipliers 57 and 59 are appliedto respective low pass filters 61 and 63 to eliminate components aroundthe i.f. frequency. The signals remaining after such filtering may beconsidered to be Doppler information signals. Each such Dopplerinformation signal comprises components which are equal in frequency tothe Doppler shift frequencies of the Doppler shifted components in theoriginal set of echo signals. In other words, the Doppler components inthe original echo signals have, in effect, been synchronously transposedor heterodyned down to a zero i.f. frequency.

Preferably, the phasing of the difference frequency signal applied toeach of the multipliers 57 and 59 is adjusted so that the nominal d.c.component of the multiplier output signal is maintained essentially at azero level. If desired, a long time constant feedback or servo loopcontrolling a phase adjuster may be used to automatically maintain sucha relationship. When this relationship is maintained and a truemultiplier is employed to perform this function which is similar toheterodyning, it can be shown the fluctuations in the source amplitudeare substantially cancelled, i.e., are reduced to second order effects.These Doppler information signals are applied to the signal processor,designated generally as 65. For convenience in the later description,the Doppler information signals are referred to as 81 and 82- In someapplications it may be desirable to reduce the dynamic range of theinput signals applied to the processor, e .g., by applying a normalizingfunction, so that the range over which the processor must operate isreduced. Accordingly, as used herein, the term Doppler informationsignal should be understood to encompass a processor input signal whichhas been normalized in amplitude as well as one which has not.

In the signal processor 65, each Doppler information signal is appliedto a respective differentiator circuit 71 and 72 which provides anoutput signal which is a differentiated version of the original Dopplerinformation signal. In other words, a time-amplitude dependenttransformation function is applied to the Doppler information signal.Each differentiated Doppler information signal is applied to one inputof a respective multiplier circuit 73 and 75. To the other inputterminal of the multiplier which receives each differentiated Dopplerinformation signal is applied the other or opposite Doppler informationsignal in undifferentiated form. In other words, there is across-combining of information obtained from the two sets of echosignals. In the embodiment illustrated, the undifferentiated informationsignal is obtained directly from the input to the signal processor butessentially the same information can also be obtained by integrating thepreviously differentiated signal. This latter method blocks extraneousd.c. level shifts.

The output signals from the two multipliers 73 and 75 are applied,respectively, to the in-phase and inverting inputs of a summing circuit,e.g. a differential amplifier, as indicated at 81. The output signalfrom the summing circuit is in turn applied to the input terminal ofan'integrator circuit 83. The time constant of integrator 83 is selectedso as to be long in relation to the expected periods of movement of theclutter points.

As both of the signals which drive the integrator 83 are generated bycross-combining signals which are derived from the two different sets ofecho signals, it can be seen that the output signals from thesemultipliers contain components having frequencies which are equal to thedifference in frequency between the corresponding Doppler shiftedcomponents in the two echo signal sets. Since the time constant ofintegrator 83 is quite long with respect to the periods of the originalDoppler shift frequencies, typically only the Doppler differencecomponents will be represented in the output signal from the integrator.In the simple case in which the only echo signal components are thosedue to a single target moving toward the apparatus at a constant speed,it can thus be seen that the output signal from the integrator 83 willconsist of a single frequency component, which frequency is equal to thedifference between the Doppler shifts imposed upon the two originaltransmitted frequencies by the velocity of the target.

The output signal from the signal processor 65 can, for example, besubjected to spectrum analysis in order to separate out components dueto targets having different radial velocities or, for intrusiondetection systems, the output signal may merely be applied to anamplitude threshold circuit which controls an alarm. Alternatively, theprocessor output signal may be subjected to further processing as beingmerely part of the information collected by a larger echo interrogationsystem of which the present apparatus is merely a component.

As noted previously, the presence of a single target moving at aconstant radial velocity would produce an integrator output signal ofsubstantially a single frequency. In other words, the instantaneousamplitude of the integrator output signal is a periodic function ofradial distance to the target. It can therefore be seen that theincremental sensitivity of the integrator output signal to small netmovements of a target varies as a periodic function of radial distance.In one sense then, the frequency difference between the two transmittedcomponents establishes a series of zones of relatively high incrementalsensitivity. These zones may be conveniently referred to as coherencyzones." If the response of the overall system is confined to a singlesuch zone, e.g. by range gating as is known in the art, the outputsignal may effectively be treated as a dc. signal having an amplitudewhich varies as a target moves through the zone.

While various other systems known in the prior art may operate incertain modes to generate an output signal which has a frequency equalto the difference between Doppler shifts, a singular advantage of thepresent apparatus is that the output signal is relatively insensitive toreciprocating or random displacements of clutter points in theenvironment while being quite sensitive to net movement of a target overthe integration period. This characteristic of the present inventionallows the apparatus to be relatively sensitive to a target which isintruding or moving across an environment under surveillance while itsubstantially ignores movingclutter such as wind-driven branches, etc.,which do not have substantial net displacements over the integrationperiod. In other words, the processor generates an integral which issubstantially independent of the path of integration. Thus, only netdisplacements are indicated.

Assuming that there are N moving energy scatterers or targets in theenvironment from which energy is being reflected back, the two Dopplerinformation signals 3 and 3 provided to the signal processor can bedescribed as follows:

N A,cos, 91 g 1 where the phases d) and are defined as 2 li c 2 25 andwhat;1;"isfiiniafizafiaearesaim o f tli i scatterer; w 1 (02 are theoriginal radiated frequencies; c is the speed of light; and R, is theradial range to the i'" scatterer.

The output signal (I) from the integrator after a time Tisthen 1 =f(gab-Quiz) where d g dt dt (5 Performing the indicated differentiationsand substitutmg,

k; 2911.11. f [a sin an) os as) 2i Sin 210) G05 n( dt Equation (6) canbe rewritten in terms of an integral over the phases Q5 and 41 asfollows:

The terms in the above integral can be divided into two sets of terms,namely the coherent," or (i j) terms and the incoherent, or (i a Jterms. The coherent terms are integrals of exact differentials and aretherefore independent of the values taken on by the variables 41 and 4)during the integration period 0 g t s r The values of the coherent termsdepend only umn the values of the various phases (b1: and d) at thelimits of the integration period. Therefore, these terms do no t tend tocontribute increasingly large random values to the integral as theintegration time r is increased. This is in contrast to the conventionalphase detector or cross correlation, in which devices the coherent termsare of the form Maj gmgzm (8) Terms of this latter form will produce anet average value which increases proportionately with (i /t).

The incoherent terms (i a j) in equation (7) are In view of theforegoing, it may be seen that several objects of the present inventionare achieved and other advantageous results have been attained.

As various changes could be made in the above construction withoutdeparting from the scope of the invention, it should be understood thatall matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense. For example, while the signal processor illustratedemploys analog functional components, it can be seen that the operationof each of these components and hence the operation of the overallprocessor can be simulated by digital computation using an appropriatelyprogrammed general purpose digital computer. Accordingly, such anembodiment should be understood to fall within the scope of the presentclaims.

What is claimed is: 1. Apparatus for processing the Doppler shiftedcomponents of respective sets of echo signals reflected back from anenvironment in response to a radiated signal comprising a pair ofadjacent frequency components, said apparatus comprising:

means for generating from said echo signals respective sets of Dopplerinformation signals having frequency components equal in frequency tothe respective Doppler shift frequencies;

means for differentiating each of said Doppler information signalsthereby to provide a respective differential signal;

means for generating a first product signal having an amplitude which isproportional to the product of the amplitude of a first of said Dopplerinformation signals with the amplitude of the differential signalcorresponding to the other of said Doppler information signals;

means for generating a second product signal having an amplitude whichis proportional to the product of the amplitude of the other of saidDoppler information signals with the amplitude of the differentialsignal corresponding to said first Doppler information signal; and

means for generating an output signal having an amplitude which variesas a function of the time integral of the difference between theamplitudes of 60 said product signals.

2, Apparatus for processing the Doppler shifted components of therespective sets of echo signals reflected back by an environment inresponse to a radiated signal comprising a pair of closely adjacentfrequency components, said apparatus comprising:

means for generating from said echo signals respective sets of Dopplerinformation signals having frequency components equal in frequency tothe respective Doppler shift frequencies;

means for differentiating each of said Doppler information signalsthereby to provide respective differential signals; means for generatinga product signal having an amplitude which is proportional to theproduct of one of said Doppler information signals and the differentialsignal corresponding to the other of said Doppler information signals;and

means for generating an output signal having an amplitude which variesas a function of the time integral of the amplitude of said productsignal. 3. Apparatus for processing a pair of Doppler informationsignals having frequency components equal in frequency to the respectiveDoppler shift frequencies contained in respective sets of echo signalsreflected back from an environment in response to a radiated signalcomprising a pair of adjacent frequency components, said apparatuscomprising:

means responsive to each of said Doppler information signal forgenerating a respective transform signal which is a time-amplitudetransformation function of the respective Doppler information signal;

means for generating a first product signal having an amplitude which isproportional to the product of the amplitude of a first of said Dopplerinformation signals with the amplitude of the transform signalcorresponding to the other of said Doppler information signals;

means for generating a second product signal having an amplitude whichis proportional to the product of the amplitude of the other of saidDoppler information signals with the amplitude of the transform signalcorresponding to said first Doppler information signal; and

means for generating an output signal having an amplitude which variesas a function of the time in tegral of the difference between theamplitudes of said product signals.

4. Apparatus for processing the Doppler shifted components of respectivesets of echo signals reflected back from an environment in response to aradiated signal comprising a pair of adjacent frequency components, saidapparatus comprising:

means for providing a difference signal having a frequency which isequal to the difference between the two radiated frequency components;

means for combining each of said echo signals with said differencesignal to generate a respective Doppler information signal havingfrequency components equal in frequency to the respective Dop pler shiftfrequencies;

means for differentiating each of said Doppler information signalsthereby to provide a respective differential signal;

means for generating a first product signal having an amplitude which isproportional to the product of the amplitude of a first of said Dopplerinformation signals with the amplitude of the differential signalcorresponding to the other of said Doppler information signals;

means for generating a second product signal having an amplitude whichis proportional to the product of the amplitude of the other of saidDoppler information signals with the amplitude of the differentialsignal corresponding to said first Doppler information signal; and

means for generating an output signal having an amplitude which variesas a function of the time integral of the difference between theamplitudes of said product signals.

5. Apparatus for processing the Doppler shifted components of respectivesets of echo signals reflected back from an environment in response to aradiated signal comprising a pair of adjacent frequency components, saidapparatus comprising:

means for generating signals having frequencies m,

m and (DI-(02 respectively, the radiated signal comprising frequencies wand m means for combining echo signals based on frequency m, with saidsignal at frequency to obtain a first Doppler i.f. signal based onfrequency co -m means for combining echo signals based on frequency 0:with said signal at frequency (0 to obtain a second Doppler i.f. signalbased on frequency m "2;

means for combining each of said Doppler i.f. signals with said signalat frequency al -(U to generate a respective Doppler information signalhaving frequency components equal in frequency to the respective Dopplershift frequencies;

means for differentiating each of said Doppler information signalsthereby to provide a respective differential signal;

means for generating a first product signal having an amplitude which isproportional to the product of the amplitude of a first of said Dopplerinformation signals with the amplitude of the differential signalcorresponding to the other of said Doppler information signals;

means for generating a second product signal having an amplitude whichis proportional to the product of the amplitude of the other of saidDoppler information signals with the amplitude of the differentialsignal corresponding to said first Doppler information signal; and

means for generating an output signal having an amplitude which variesas a function of the time integral of the difference between theamplitudes of said product signals.

6. Apparatus as set forth in claim 5 wherein each of said signalcombining means comprises a multiplier.

1. Apparatus for processing the Doppler shifted components of respectivesets of echo signals reflected back from an environment in response to aradiated signal comprising a pair of adjacent frequency components, saidapparatus comprising: means for generating from said echo signalsrespective sets of Doppler information signals having frequencycomponents equal in frequency to the respective Doppler shiftfrequencies; means for differentiating each of said Doppler informationsignals thereby to provide a respective differential signal; means forgenerating a first product signal having an amplitude which isproportional to the product of the amplitude of a first of said Dopplerinformation signals with the amplitude of the differential signalcorresponding to the other of said Doppler information signals; meansfor generating a second product signal having an amplitude which isproportional to the product of the amplitude of the other of saidDoppler information signals with the amplitude of the differentialsignal corresponding to said first Doppler information signal; and meansfor generating an output signal having an amplitude which varies as afunction of the time integral of the difference between the amplitudesof said product signals.
 2. Apparatus for processing the Doppler shiftedcomponents of the respective sets of echo signals reflected back by anenvironment in response to a radiated signal comprising a pair ofclosely adjacent frequency components, said apparatus comprising: meansfor generating from said echo signals respective sets of Dopplerinformation signals having frequency components equal in frequency tothe respective Doppler shift frequencies; means for differentiating eachof said Doppler information signals thereby to provide respectivedifferential signals; means for generating a product signal having anamplitude which is proportional to the product of one of said Dopplerinformation signals and the differential signal corresponding to theother of said Doppler information signals; and means for generating anoutput signal having an amplitude which varies as a function of the timeintegral of the amplitude of said product signal.
 3. Apparatus forprocessing a pair of Doppler information signals having frequencycomponents equal in frequency to the respective Doppler shiftfrequencies contained in respective sets of echo signals reflected backfrom an environment in response to a radiated signal comprising a pairof adjacent frequency components, said apparatus comprising: meansresponsive to each of said Doppler information signal for generating arespective transfoRm signal which is a time-amplitude transformationfunction of the respective Doppler information signal; means forgenerating a first product signal having an amplitude which isproportional to the product of the amplitude of a first of said Dopplerinformation signals with the amplitude of the transform signalcorresponding to the other of said Doppler information signals; meansfor generating a second product signal having an amplitude which isproportional to the product of the amplitude of the other of saidDoppler information signals with the amplitude of the transform signalcorresponding to said first Doppler information signal; and means forgenerating an output signal having an amplitude which varies as afunction of the time integral of the difference between the amplitudesof said product signals.
 4. Apparatus for processing the Doppler shiftedcomponents of respective sets of echo signals reflected back from anenvironment in response to a radiated signal comprising a pair ofadjacent frequency components, said apparatus comprising: means forproviding a difference signal having a frequency which is equal to thedifference between the two radiated frequency components; means forcombining each of said echo signals with said difference signal togenerate a respective Doppler information signal having frequencycomponents equal in frequency to the respective Doppler shiftfrequencies; means for differentiating each of said Doppler informationsignals thereby to provide a respective differential signal; means forgenerating a first product signal having an amplitude which isproportional to the product of the amplitude of a first of said Dopplerinformation signals with the amplitude of the differential signalcorresponding to the other of said Doppler information signals; meansfor generating a second product signal having an amplitude which isproportional to the product of the amplitude of the other of saidDoppler information signals with the amplitude of the differentialsignal corresponding to said first Doppler information signal; and meansfor generating an output signal having an amplitude which varies as afunction of the time integral of the difference between the amplitudesof said product signals.
 5. Apparatus for processing the Doppler shiftedcomponents of respective sets of echo signals reflected back from anenvironment in response to a radiated signal comprising a pair ofadjacent frequency components, said apparatus comprising: means forgenerating signals having frequencies omega 1 , omega 2 and omega 1-omega 2 respectively, the radiated signal comprising frequencies omega 1and omega 2 ; means for combining echo signals based on frequency omega1 with said signal at frequency omega 2 to obtain a first Doppler i.f.signal based on frequency omega 1- omega 2; means for combining echosignals based on frequency omega 2 with said signal at frequency omega 1to obtain a second Doppler i.f. signal based on frequency omega 1- omega2; means for combining each of said Doppler i.f. signals with saidsignal at frequency omega 1- omega 2 to generate a respective Dopplerinformation signal having frequency components equal in frequency to therespective Doppler shift frequencies; means for differentiating each ofsaid Doppler information signals thereby to provide a respectivedifferential signal; means for generating a first product signal havingan amplitude which is proportional to the product of the amplitude of afirst of said Doppler information signals with the amplitude of thedifferential signal corresponding to the other of said Dopplerinformation signals; means for generating a second product signal havingan amplitude which is proportional to the product of the amplitude ofthe other of said Doppler information signalS with the amplitude of thedifferential signal corresponding to said first Doppler informationsignal; and means for generating an output signal having an amplitudewhich varies as a function of the time integral of the differencebetween the amplitudes of said product signals.
 6. Apparatus as setforth in claim 5 wherein each of said signal combining means comprises amultiplier.